The embodiments described herein relate generally to the field of non-destructive evaluation (NDE) using thermographic images. Infrared (IR) flash (or pulsed) thermography is an example of a technique for NDE used in the inspection of thin materials such as laminated or bonded composites in the aerospace industry. IR flash thermography is used to detect delamination-like anomalies, although other anomalies, such as surface cracks, may be detected.
An example of hardware equipment for an IR flash thermography system comprises a flash lamp (source of light/heat), a flash hood or housing, a flash power supply/trigger unit, a flash duration controller, an IR camera for capturing video images, data acquisition electronics, and a computer. The computer may be used for controlling the flash trigger, for acquiring video data from the IR camera, for displaying data, and for post-processing of the acquired data.
In one example of an NDE technique using IR flash thermography, a single sided or reflection technique is used wherein the flash lamp (heat source) and the IR camera (detector) are on the same side of a test object undergoing inspection. A plate is provided as the test object with a round delamination in the center. After applying heat to the top surface of the test object by triggering the flash lamp, the top surface area surrounding the anomaly cools faster than the top surface (footprint) area above the anomaly. The IR camera captures a sequence of images of the surface temperature in terms of pixel intensity and represents the anomaly as a hot spot (e.g., an area warmer than the surrounding area or the reference region of interest (ROI)). The hot spot is about the size and shape of the anomaly footprint. Relative pixel intensity, i.e., the difference in pixel intensity between the hot spot (measurement ROI) and the surrounding area (reference ROI), varies with the post-flash time. Deeper anomalies appear in the IR video data at later times compared to the near surface anomalies. After the initial appearance of an anomaly in the IR video data, the relative pixel intensity continues to increase with time. The relative pixel intensity of the anomaly reaches a peak at a certain time, and then the relative pixel intensity decays until the temperature of the indication area and the temperature of the surrounding area become equal.